1. Field of the invention
The present invention generally relates to filter chips, and more particularly, to a filter chip having multiple resonators arranged in a ladder formation and a filter device equipped with such a filter chip.
2. Description of the Related Art
A band-pass filter is employed at a radio frequency (RF) stage of a portable or mobile wireless apparatus such as a cellular phone. Generally, such a band-pass filter is required to have a degree of suppression to out-of-pass-band frequencies as high as 20 dB to 40 dB. Conventionally, the band-pass filter widely used has a structure in which multiple surface acoustic wave resonators are arranged in a ladder formation. Various ways to improve the degree of suppression to the out-of-pass-band frequencies have been proposed in connection with the ladder-type filter.
For example, it is proposed to increase the number of stages of resonators in the ladder arrangement. It is also proposed to increase the ratio Cp/Cs where Cs denotes the electrostatic capacitance of the resonator arranged in a series arm of the ladder arrangement, and Cp denotes the electrostatic capacitance of the resonator arranged in a parallel arm thereof. However, these proposals increase insertion loss in the pass band.
Another proposal based on the above consideration is known. According to this proposal, inductance elements are added to the resonators in the parallel arm in order to reduce the insertion loss in the pass band and simultaneously enhance the effect of suppression to the out-of-pass-band frequencies. More particularly, a wire is used to connect a ground electrode in each of the parallel-arm resonators in the ladder arrangement and a ground pad on a package (see, for example, Japanese Unexamined Patent Publication No. 5-183380).
However, it is not expected that the use of wire for making a connection between the filter chip and the package to thus obtain a desired inductance provides the sufficient degree of suppression to the frequencies out of the pass band in flip-chip or face-down mounting in which bumps having a smaller inductance than that of the wire are used as a connecting member for making an external connection.